Mitigation of coarse-grained heat-affected zone embrittlement via microstructural tailoring of martensite–austenite constituents for high-heat-input welding

Low-temperature toughness of coarse-grained heat-affected zone (CGHAZ) is investigated with varying C and Nb contents, focusing on mitigating CGHAZ embrittlement without sacrificing the mechanical performance of as-rolled plates. Higher Nb promoted deformation-induced ferrite transformation in as-rolled plates, whereas in the CGHAZ, lower C produced degenerated pearlite instead of martensite–austenite (MA) constituents. Such phase transformations reduce the size and volume fraction of MA constituents, decrease potential crack-initiation sites, and enhance crack-propagation resistance. Consequently, the 6C–2Nb steel exhibited lower ductile-to-brittle transition temperatures of −73 °C and −30 °C in as-rolled plates and CGHAZ, respectively, compared to −46 °C and −16 °C for 7C–1Nb steel. This study proposes a microstructural approach to overcome the traditional ultimate tensile strength–toughness trade-off while preserving the matrix structure.